1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes
generating means for generating electromagnetic fields which are required so as to form MR images, PA1 transmission means for transmitting data signals, PA1 a first part of said means being accommodated in a Faraday cage whereas another part is accommodated outside the Faraday cage, PA1 said first and second parts being interconnected via connection leads which are fed through a wall of the Faraday cage by means of a feedthrough device which includes feedthrough filters.
2. Description of Related Art
A feedthrough device for feeding connection leads through a wall of a Faraday cage by means of feedthrough filters is known from the published UK patent application No. GB 2 184 293.
Generally speaking, a magnetic resonance imaging apparatus includes a coil system for generating a steady magnetic field in a measuring space of the apparatus, a gradient coil system for generating a magnetic gradient field in said measuring space, and RF coils for generating an RF magnetic alternating field. Because some of these coil systems (notably the gradient coil system) involve comparatively large currents (of the order of magnitude of some hundreds of A) and high voltages (of the order of magnitude of from several hundreds of V to kV) and because, moreover, very weak and noise-sensitive signals must be detected in such an apparatus, the measuring space of the apparatus and the associated coils are accommodated in a Faraday cage in order to shield external interference fields and to counteract the emergence of electromagnetic fields generated by the MRI apparatus itself. A part of the equipment of the MRI apparatus, however, is accommodated outside the Faraday cage. This holds for example, for the amplifiers for controlling the gradient coil system, for the data processing computer which serves to reconstruct the desired image from the measuring data obtained, for the control computer which serves to control the transport of the control signals required for generating the various measuring data, and for various power supply units which serve to supply the equipment inside the Faraday cage with energy.
In order to establish the necessary connections between the equipment inside the Faraday cage and that outside this cage there may be provided a feedthrough device which is arranged in the wall of the Faraday cage and is provided with feedthrough filters. Depending on the nature of the connection to be fed through, these feedthrough filters may take various forms.
For example, a glass fiber which is electrically insulating per se may be fed through a conductive tube having a length-to-width ratio which is larger than four. The attenuation (measured in dB) of such a tube amounts to 32 times the length-to-width ratio. When the appropriate length is chosen, adequate attenuation of external interference signals is then achieved in such a tube.
In order to prevent electrically conductive leads from transporting the interference signals generated outside the Faraday cage to the equipment arranged within the Faraday cage, the feedthrough device should satisfy a variety of requirements in respect of electromagnetic shielding. A conductor for transporting RF signals, for example should be constructed as a coaxial conductor provided with an electromagnetically suitably sealed cladding. This is achieved by constructing the cladding as a mechanically suitably tight tube; this coaxial conductor then becomes rigid and hence difficult to handle.
For the connection leads for the transmission of data signals (which need not transmit signals with a frequency content above 100 kHz), the requirement is imposed that the feedthrough filters must be low-pass filters and at 10 MHz an attenuation of 100 dB is required, corresponding to 18 dB per octave. Usually a so-called H filter is used for this purpose, i.e. a low-pass filter which consists of a first capacitor connected parallel to the input of the filter, a self-inductance between an input conductor and an output conductor, and a second capacitor connected parallel to the output of the filter.
The cited patent application GB 2 184 293 discloses a feedthrough device in which a number of connection leads is fed through a wall of the Faraday cage by means of a feedthrough device provided with feedthrough filters. The feedthrough filters described therein are filter elements, each of which includes one II filter, so that these filters offer a suitable attenuation for comparatively high frequencies. However, if these filters were constructed for use in an MRI apparatus, they would have to conduct very large currents (of the order of magnitude of some hundreds of A) so that they would be very voluminous and expensive. The volume of these filters would be increased further due to said required attenuation of 18 dB/octave at high frequencies, so that they would have to be constructed so as to be rotationally symmetrical, which in its turn would lead to additional requirements as regards the construction and a further increase of the volume of the filters.